1. Technical Field
The present invention relates to integrated circuits and in particular to MOS integrated circuits. Still more particularly, the present invention relates to switch circuitry for switching between power supplies.
2. Description of the Related Art
In some situations, it is desirable to provide retention of data in integrated circuits such as memory devices. A number of circuits are commercially available for retaining data in SRAMS when power is removed. An example of one such device may be found in U.S. Pat. No. 5,099,453, entitled Configuration Memory For Programmable Logic Device, U.S. Pat. No. 4,713,555, entitled Battery Charging Protection Circuit; U.S. Pat. No. 4,122,359, entitled Memory Protection Arrangement, and U.S. Pat. No. 4,451,742, entitled Power Supply Control For Integrated Circuit. These devices are often known as xe2x80x9czero power circuitsxe2x80x9d. Typically, in a zero power circuit, the contents of the circuit are protected in the event that the power supply voltage to that circuit drops below some predetermined or selected threshold voltage i.e., the voltage of a secondary power supply. This protection may be accomplished by switching the circuit from a primary power apply to a secondary power supply, typically an internal battery, when the voltage of the primary power supply drops below that of the secondary power supply.
Power control circuits exist which provide automatic sensing of a primary power source voltage. These power control circuits provide for automatic switching to a secondary power source when the primary power source voltage drops below a predetermined threshold voltage.
Typically, a comparator is employed to compare the voltage of the external power source with the voltage of a predetermined threshold, which is typically the voltage of the battery power supply. When the external power supply voltage drops below the voltage of the battery power supply voltage, the circuit typically switches the integrated circuit to the battery power supply from the external power supply. Such a system works well for integrated circuit systems that employ 5.0 volt power supplies. Problems, however, occur for integrated circuit systems running on power supply voltages at 3.0 volts. In particular, these powers supply voltages may range about from 2.7 to about 4.0 volts. As a result, at various times the external power supply voltage may drop below that of the battery supply power voltage. Therefore, it would be advantageous to have a method and apparatus for switching power supplies that does not rely on the voltage of the battery power supply as a reference voltage to switch power supplies.
The present invention provides an apparatus and method for switching between two power supplies, a primary power supply and a secondary power supply. The present invention generates a first reference voltage using the voltage of the primary power supply and the secondary power supply, wherein the primary power supply voltage is variable. The present invention also generates a second reference voltage based on the voltage of the primary power supply. The first and second reference voltages each have a different slope and the crossing point between these two reference voltages indicate that a switch between the primary power supply and the secondary power supply should occur.
Typically, the primary power supply is an external power supply to a integrated circuit, and the secondary power supply is a battery power supply that is used when the primary power supply voltage drops below some pre-selected level. The primary power supply voltage is reconnected to the integrated circuit when it exceeds the preselected level. These preselected levels are selected by adjusting the crossing point between the first and second reference voltages according to the present invention.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.